I can't reproduce this. The only way I could see it happening internally is if there's a user with an empty name, which generally shouldn't be possible.

However, an add-on could trigger this -- even an add-on that adds a column called "username" to a table like xf_forum. I'd try first to disable add-ons, though the issue may still persist if it's a column conflict. (If it's a conflict, the author would need to change the column name.)